Condensation in Bivy

Chuck,

You are not alone. Last winter we started receiving a lot of reports from readers about condensation in Epic bivies in the winter. They are continuing again this year.

This fall we embarked on a research project to try to figure out what’s going on at these temperatures – 15 to 35 degrees F usually – that is causing the interior of Epic bivies to get so wet when other, more waterproof bivies, are remaining relatively dry with only frost.

We will be publishing the data and results in a report this spring, but here are a few highlights that we’ve learned from our lab and field tests:

– With a waterproof fabric that is able to pass vapor via convection (eVENT), vapor either passes directly through the fabric to the outside or condenses on the relatively cold inner surface of the fabric and immediately forms frost. The frost doesn’t interfere with subsequent breathability, and the sack continues to perform well at passing vapor.

– With Epic, condensation is forming in the interstices of the fabric itself, causing condensation and frost to PLUG the pores of the fabric. Breathability is thus reduced, and interior fabric temperature is raised slightly in some circumstances. As such, over time, the ability of the fabric to breathe decreases over time and condensation buildup thus increases.

We have some neat lab data and microscopic photos that show the process of condensation and frost plugging up the interstices of Epic, it’s a pretty fascinating phenomenon. It occurs to some extent with other water resistant “highly breathable” fabrics like uncoated nylons, but not the extent to which it occurs in Epic, probably because the breathability of Epic is just low enough to cause condensation to form in the fabric itself.

This (condensation in the interstices that decreases breathability) does not seem to occur in eVENT or even Gore-Tex (which has its own set of problems), because the pores themselves are too small for water molecules to coelesce (condense) to form substantial liquid/frozen structures that block pores.

This lab data, and these theories, seem to be supported well by field data taken from several different bivy products, by several different people, in quite a wide range of conditions.

If anyone out there is further interested in doing some controlled tests with their bivy sacks (any material or design welcome), please drop us a line and we’ll get you involved with the study.

Ryan

Ryan-

Whoa! That last message is a bomb that’s going to have fallout throughout the outdoor gear industry!

Here in North Idaho, that 15-35 degree range covers much of the entire year. I look forward to your article in the Spring. Two questions for you now, though:

1) Do you expect similar condensation problems to occur in garments, sleeping bags, and tents? Or, is there something unique about the bivy microclimate which is contributing to this?

2) What happens below 15 degrees? Does Epic compare more favorably to other alternatives here?

Thanks!

-Mike

Ryan,I would love to hear the answers to Michael’s questions because I’m looking to buy a new sleeping bag with an epic shell.

Richard,

Though most of my experience in temperatures between 5-35 degrees F have been in my vehicle, I have never experienced a problem with condensation in my eVENT Feathered Friends Lark sleeping bag.

OK, here are my answers to Mike’s questions:

1) Do you expect similar condensation problems to occur in garments, sleeping bags, and tents? Or, is there something unique about the bivy microclimate which is contributing to this?

There is something unique going on. We don’t see remarkable differences in garments, and in fact, Epic is perceived to be more breathable in a garment configuration than eVENT. As for tents, we are getting mixed results with Epic, but are still finding that it’s not completely waterproof in heavy rain, so I think it’s best application is in winter conditions. However, the Epic tents on the market aren’t particularly wind resistant (lack of fabric resiliency in the epic makes them a weak point in tent structure) so an eVENT tent makes more sense when serious storm conditions are expected.

2) What happens below 15 degrees? Does Epic compare more favorably to other alternatives here?

Not in bivies. We are finding that ice is still clogging interstices in the fabric and reducing breathability.

I suspect that what we may be seeing regarding the differences in the performance of Epic in clothing and bivies may in part have to do with the differences in temperature (partially due to distance from the skin surface), convection, conduction, and very possibly the number of air surfaces between the layers of fabric.

Responding to Ryan– As for the attributes/drawbacks of Epic in tents,I can only speak from my own experience.

I recently got back from a Trinity Alps ski backpack
( think of a more northerly version of the Sierra) . We tended to stay high and on ridges above timberline. We were fairly blasted by last weekend’s winter storm with winds peaking above 60 mph and significant snowfall. My partner and I were using a BD Lighthouse Epic tent in a rather exposed site (but strategically located by some wonderful backcountry runs). I thought it handled the winds as well as any light weight 4 season tent
I’ve used ( including a long series of single and double wall tents).Long term ability to do so remains untested but so far, no fabric or pole failure. Wind penetration was no more than I’ve experienced in Bibler or ID(not eVent) tents and condensation perhaps less.Condensation was minimal except when once melting some snow on a stove (didn’t use a vestibule).Then again,cooking in a tent in Winter has always yielded quite a bit of condensation in my experience, including double wall tents.Snow caves,properly vented,are better for
such winter shenanigins,but we were lazy.
Water penetration in my Lighthouse has not been an issue for me–certainly not so far this Winter. It has been through several downpours of several hours duration last fall without any significant leakage–and that was through an unsealed seam.
Perhaps my expectations for Epic water proofness were never stratospheric, but I have been pleasantly surprised at just how well it has kept the
water out.
All this being said, I look forward to trying an eVent tent that can fit my over 6 foot frame and can
compete in weight with the BD tents . Will this be possible down the line ?

Ok, so maybe it was more like a loud thump than a bomb…

Thanks very much for the reply, Ryan. Thank you also to Richard and Kevin for your comments.

I’ve got a totally untested theory (aka pure wild speculation) I wanted to put forward which might support Ryan’s findings:

1) For tents, the condensation issue may not be as critical because there is a greater surface area to clog with ice before breathability really becomes compromised. Also, unlike a bivy, you can avoid touching the condensed fabric surface on a tent to prevent wetting your bag.

2) For Epic garments and sleeping bags, I would guess that comparatively more condensation would occur in the insulation before reaching the shell fabric as there is no opportunity for the temperature gradient to cross the dewpoint between the bag and the bivy shell. Also, as the insulation is directly in contact with the Epic, it might wick some of the moisture away from the fabric back into the bag. Condensation would still occur inside the bag, but the shell breathability would not become as compromised as with a bivy. With a DWR treated bag inside a bivy, any condensation that occurs outside the bag is going to freeze directly on the Epic fabric. Whereas an Epic-shelled jacket or bag provides an opportunity for moisture to disperse in the insulation without clogging the shell.

3) I think an interesting test would be to use a wicking overbag (perhaps silk?) between a sleeping bag and Epic bivy and see if it improves the situation — of course, that would put you up in the weight range of an eVent bivy, avoiding the issue altogether.

-Mike

I have a GoLite jacket made from EPIC (2002 season), that is a fine wind breaker, but a very poor water resistant jacket and seems to breath only a very limited basis. It won’t take even a very light sprinkle without soaking through quickly. And, with very little exertion, it heats quickly.

Given my experience with EPIC in this jacket, I question its use as a sleeping bag fabric, although I know Feathered Friends offers it. They also offer e-Vent, which I’m beginning to think may be a far superior fabric for sleeping bags as compared to EPIC.

I see a "study" mentioned. Did one ever take place?